The magical world of Microsoft Windows

Good afternoon,
Newbie in the LDAP world, I have a problem I suppose someone has already
had. I have a Linux box providing WebMail access to a Microsoft Exchange
2000 Server in an Active Directory domain. I do have an Apache 1.3.23
(mod_ssl 2.8.7), with PHP 4.1.1 compiled with IMAP (imap-2000c) and LDAP
(openldap-2.0.21) support, running IMP 3.0 and Turba 1.0 under Horde 2.0,
and last, a mysql database (3.23.49a).
Well, at this time, users can connect through HTTPS to imp from the Internet
and check their mail (stored in the Exchange 2000 Server), and they can
create custom addressbook stored in the mysql database.
Now, and I suppose it is highly possible, I want to create a "link" to the
Address Book stored in Active Directory, so people can have an unified and
complete address book.
For now, I want to ask Active Directory to give me the list of the users,
and their email addresses. I should find this list under :
ldap://1.2.3.4/DC=my-domain,DC=com and I suppose `ldapsearch -LLL
"(p=MYDOMAIN)" cn` but that does not work, and the other idea I had
`ldapsearch -LLL "(o=Exchange)" cn` does not work too.
Anyone has an idea ?